The present invention relates to the field of seismic source detection in geophysical operations. More particularly, the invention relates to a seismic source sensor for detecting operation of a seismic source in water or on land.
Seismic energy sources transmit acoustic energy into land or water surfaces which propagates downwardly through underlying geologic formations. The interfaces and features of subsurface geologic formations reflect a portion of the acoustic source energy which moves upwardly to the land or water surface where it is recorded. On land, explosives, vibrators, and other devices generate the seismic energy. The receiver elements are deployed at or near the surface of the Earth. In water, the acoustic source energy is generated using air guns or other water displacing devices. The seismic cables and associated receiver elements are towed behind marine seismic vessels or are laid on the seafloor.
The reflected seismic source energy is detected with hydrophone and geophone sensors located at a distance from the seismic source. Pressure sensitive type films have been used in U.S. Pat. No. 4,789,971 to Powers et al. (1988), which disclosed an acoustic hydrophone formed with polyvinylidene flouride (PVDF). Another acoustic sensor was disclosed in U.S. Pat. No. 5,361,240 to Pearce (1994) wherein a flexible piezoelectric film was wrapped several times around a mandrel. A hollow space between the film and the mandrel provided a pressure compensation chamber to permit activation of the film. Another acoustic sensor was disclosed in U.S. Pat. No. 5,774,423 to Pearce et al. (1998) wherein a flexible piezoelectric material was encapsulated within a segmented housing. Two or more clam shell type housings were fastened to a cable to form a hydrophone and a hollow space permitted flexure of the piezoelectric material.
Because of the depth of the geologic formations under investigation, hydrophone and geophone sensors are typically located at a significant distance from the seismic source. Therefore, the energy detected by the hydrophone and geophone sensors represents a xe2x80x9cfar-fieldxe2x80x9d energy pulse resulting from transmission of the seismic source. The transmitted seismic pulse obtained from a point on or near the source does not accurately characterize the far-field signal observed at the seismic sensors. Conventional sensors used to detect the energy output of the source are located at discrete points on or near the energy source. Because the sensors occupy discrete positions they cannot sense the total energy output of the energy source. Current practice involves estimates of the far-field signal from either the weighted sum of the vibrator and reaction mass signals, known as ground force, of a land vibrator or near-field hydrophone measurements made near a marine source. Although such estimates represent an improvement over previous methods, such estimates do not reflect the total energy output of the seismic source and do not adequately represent the far-field signature of the source.
A need exists for a system capable of predicting the far-field acoustic signal initiated by a seismic source. This prediction can be made by assessing movement of the seismic energy source during source activation. However, tests of seismic sources such as slotted cylinders, vibrators and similar sources demonstrate that the active components of such devices do not move uniformly during activation. For the seismic frequency range of interest, generally between two and one hundred-twenty Hertz, land sources exhibit a very complex motion. The source baseplate often flexes due to the force being used to vibrate the Earth surface. The source may rock back and forth due to uneven ground. Resonances within the source structure may be transmitted through the vibrator baseplate into the ground. Because all of these motions cause energy to be input into the ground, such motions should be represented in the far-field signature so that they are treated as part of the signal rather than undesirable noise.
The current practice of utilizing discrete sensors to measure the source output ignores these energy sources. The acoustic pulse of a wavetrain produced by a marine seismic source depends on the total displacement of water by the transducer. Local variations in component motion have some effect on the far-field acoustic signal. The total range of displacement of the seismic source actuator affects low frequency signal generation. Attempts to characterize the far-field acoustic signal from such marine seismic sources have required numerous sensors on or near the source exterior and have not been possible with a single sensor.
Various systems have been proposed to predict far-field signal transmission from a seismic source. For example, one system was described in U.S. Pat. No. 4,184,144 to Rickenbacker, which measured the output force of a seismic vibrator. U.S. Pat. No. 4,646,274 to Martinez disclosed a method and apparatus for correcting distorted seismic data. U.S. Pat. No. 4,670,863 to Sallas and Trevino disclosed a vibrator seismic source having a distortion limiting control system. U.S. Pat. No. 4,750,157 to Shei disclosed a seismic vibrator impedance determination and compensation system. U.S. Pat. No. 4,755,976 to Edelmann disclosed a method and apparatus for controlling and analyzing energy transfer to soil, and U.S. Pat. No. 5,790,473 to Allen described a high fidelity vibratory source seismic method for use with a plurality of seismic sources.
Conventional source sensors do not effectively measure the far-field performance of a seismic source. The performance of land vibrators or other seismic sources is difficult to measure due to the complex nature of the movements of the source components. On land, prior efforts to characterize vibrator ground force at the baseplate-soil interface have been indirect. Single sensors on or near the moving components of the source have been used to infer the source output. These sensors could not, however, accurately measure such force because of baseplate flexibility and near-surface inhomogenieties. Similarly, accelerometer data from a relatively small number of points on the baseplate have been used to infer the force exerted on the ground, however such accelerometer data do not accurately represent the baseplate motion.
Accelerometers and other sensors located at the seismic source detect near-field representation at distinct points and do not accurately characterize the far field acoustic energy pulse. Previous attempts to directly measure the total ground force signal generated by the source have involved large, heavy force tiles which are difficult by their nature to use in a production mode.
The performance of air guns, slotted cylinders and other marine acoustic seismic sources is extremely difficult to measure, and the configuration and movement of each source often generate different source energy for each seismic event.
Because seismic sources generate different signals during each seismic event, differences in the far-field data will be affected by such source variations and complicate the assessment of reflected signals. Prediction and adjustment for variations in the source events is essential to accurate assessment and characterization of the reflected signals, and a need exists for an improved system capable of assessing seismic source operation.
The present invention provides an acoustic source sensor system for use in seismic operations. The system comprises a seismic source activatable to generate an acoustic energy event, a pressure sensitive film responsive to the acoustic energy event which is disposed at least partially about the source, a connector engaged with the film for detecting response of the film to the acoustic energy event and for transmitting an electric signal representing the film response, and a controller engaged with the connector for receiving the electrical signal.
One embodiment of the invention comprises a seismic source activatable to generate an acoustic energy event in water, a pressure sensitive film disposed at least partially about the source, a connector engaged with the film for detecting response of the film to the acoustic energy event and for transmitting an electric signal representing the film response, and a controller engaged with the connector for receiving the electrical signal.
Another embodiment of the invention comprises a seismic source activatable to generate an acoustic energy event discharging energy into soil, a pressure sensitive film disposed between the source and the soil, a connector engaged with the film for detecting response of the film to the acoustic energy event and for transmitting an electric signal representing the film response, and a controller engaged with the connector for receiving the electrical signal.